The dark side of the Moon

While the Earth aligned itself with the Moon and the Sun, initiating the shortest total lunar eclipse of the 21st century, we were in the cold room in the middle of the Pacific, half-way through sieving of the samples from the previous epibenthic sledge deployment and not spending too much thought (or just very little) on what was just happening in the sky above us, but rather on the bottom of the sea four-and-a-half kilometres underneath.

It’s been widely appreciated that we know more of the universe, the surface of the Moon and the number of the stars in the Milky Way than about life in the deep sea. How many deep-sea species are there, how are they distributed and how do they find each other to reproduce, if there is permanent darkness and also not so many suitable mates around? These are some of the topics we deal with as deep-sea biologists in general. Though, facing mining and related impacts on the deep seafloor of the CCZ and its residents there is a more pending question: how can we preserve its biodiversity to make it available for the future generations?

Given that manganese nodules have formed over millions of years it will probably take tremendously long after mining for the seafloor to return to previous conditions. That is why preservation areas will need to be designated where no mining will occur and from where organisms can reconquer disturbed areas. So can all species be found everywhere or rather does each species have a distinctly limited distribution? The latter would, for instance, tell us where preservation areas would need to be placed relative to the mining site.

From the little we know about life in the manganese nodule belt we found that some species have larval stages, which can be dispersed via oceanic currents. Others such as isopod and tanaidacean crustaceans brood their young and carry them around in a ventral brooding pouch. Furthermore numerous tanaidaceans build their tubes inside which they live for presumably a whole life. The juveniles, that are released from the brood pouches, stay in maternal tubes for a few days. Once they are ready for an “independent” life they build their own tube next to the maternal one. Nevertheless, this developmental mode was described based on the observation of shallow-water tanaidaceans and we can only anticipate that deep-sea tanaidaceans may behave in the same way.

Isopods and tanaidaceans are the ones, we are most interested in. And those are the ones, which are likely to be particularly sensitive to the destruction of their surroundings, as their home range may be very restricted. This, however, is just part of the story, as some CCZ isopods have been shown to live several hundred kilometres apart – a short hop compared to the distance to the Moon, though still a giant leap for deep-sea crustaceans only few millimetres in size, some of which may not even crawl or swim. How they disperse and what their actual distributional range is, remains obscure. This is also because most of the species we find are new to science and they all need to be described and named. Until now only a small fraction of the CCZ has been sampled and certainly many species are yet undiscovered. But this is why we are here for, and every sample and every individual we look at adds another jigsaw piece to a giant and fascinating puzzle.

So coming back to the beginning: there are about 200 billion stars in the Milky Way, but how many species live in the CCZ? We still owe you an answer on this one!